HELIUM ISOTOPIC COMPOSITION IN ALKALINE INTRUSIONS OF THE HOVSGOL AREA, NORTHWESTERN MONGOLIA

Abstract

The helium isotope composition of fluid inclusions has been studied in magnetite from alkaline and subalkaline intrusions of the Hovsgol area in NW Mongolia. The measured content of 4He is in the range from 6.6 × 10–7 to 114 × 10–7 см3/г. The 3He/4He isotopic ratio of most samples varies within 0.23–0.59 Ra and may indicate the presence of helium from different sources in the magmatogenic fluid. The largest amount of mantle He (2.51 Ra) is hosted by the magnetite of the subalkaline gabbro. During the generation of primary magma, which probably involved either SCLM-type material or a plume-like reservoir less enriched in 3He, the mantle component accounted for approximately 40–60%. In the evolution of foid and alkali-syenite melts, its share did not exceed ~10–15% due to mixing with crustal radiogenic He. We assume that the intrusions interacted with fragments of Precambrian accretionary-collisional complexes of the Tuva-Mongolian terrane. Such mixing of He isotopes in melts and fluids may be regarded as circumstantial evidence supporting the development of mantle magmatism at the active continental margin.